Drilling fluid, known as mud, performs several different functions essential to drilling an oil or gas well and enhances the overall efficiency of the drilling operation. Drilling fluid is used, for instance, to cool and lubricate the drilling tool, to reduce friction between the bit and the well bore, to control subsurface pressure in the well bore, to lift the drill cuttings and carry them to the surface, and to clean the well bore and drilling tool.
The major component of drilling fluid is its base fluid, which may be aqueous based, hydrocarbon based or an emulsion. Aqueous or water based drilling fluids are used frequently in the industry and the base fluid may be either fresh water or salt water. Hydrocarbon or oil based drilling fluids are also commonly used, as are invert emulsions (i.e. water in oil). To complete the drilling of complex wells in Western Canada and throughout the world, many operators have turned to hydrocarbon based drilling fluids. Today's drilling engineer has concerns that range from the complexity of well design to stabilizing the well bore under extreme hole conditions. Drilling complex wells creates costly operations and drilling with hydrocarbons allows operators some margin of confidence in completing drilling objectives.
When drilling engineers choose to drill with hydrocarbons there are still some issues for concern, the primary concerns being the cost of the refined base oil, which is directly tied to the price of world oil, and seepage losses or, in more severe cases, total losses, which rapidly increase the cost of drilling. Both the nature and the composition of drilling fluid contributes to the high cost of drilling, in terms of the materials, composition, chemical formulations, and manufacturing processes involved. The enormous volume of drilling fluid required to complete each operation also contributes to the high cost
In the process of drilling a well, the drilling fluid is pumped into the well bore through the drill pipe and exits through nozzles in the drill bit. The drilling fluid then flows back to the surface through the annulus, which is the space between the drill pipe and the wall of the well bore. Back at the surface, the cuttings are removed and the mud is generally pumped back to a fluid tank where it can be reused and/or treated if necessary. The drilling fluid system is typically designed as a loop with the drilling fluid continually circulating as the drill bit rotates.
During the drilling operation, a portion of the drilling fluid may filter into the permeable subterranean formation surrounding the well bore and is therefore not returned to the surface for recirculation. This lost portion of drilling fluid that flows into the formation is generally referred to in the industry as lost circulation. Any lost circulation experienced has a significant economic impact on the operation. Lost circulation, particularly of hydrocarbon based drilling fluids, may also have a negative impact on the environment.
Lost circulation may occur in the form of seepage losses or fluid losses.
Seepage losses occur when whole fluids are lost to formations during drilling. This can occur, for example, when solids in the drilling fluid system are not large enough to serve as effective bridging agents for the porous or fractured formations. Mild to moderate seepage losses do not result in total loss of drilling mud to the formation but these losses nonetheless have a significant impact on the cost of drilling. Severe losses may be experienced in highly porous or fractured formations. The amount of seepage loss experienced depends on the structure and permeability of the formation being drilled. An area of a formation where seepage loss occurs is commonly referred to as a loss zone.
In contrast to seepage loss, fluid loss refers to loss of base fluid to formations where a filter cake is formed or where solids cannot pass, i.e. where only the base fluid is lost. Properties that can be controlled in regard to fluid loss are the amount of filtrate and the thickness of the filter cake created, for example, by adding various polymers or desirable fine solids to the drilling fluid.
When programming a well to be drilled with hydrocarbons, seepage losses are usually estimated based on the size of the hole in millimeters over a 100 meter hole section. An example of this would be as follows: 200 mm hole section 3-4 m3 of losses per 100 meters of new hole drilled, 222 mm hole section 4-6 m3 of losses per 100 meters of new hole drilled, 311 mm hole sections >6 m3 of losses per 100 meter of new hole drilled.
Seepage losses can be reduced, by varying amounts, by adding foreign solids to the fluid. The method is to plug or build a mat of material in, on, or near the well bore to seal off seepage losses between the drilling fluid and underground formations. Various products have been added to drilling fluids over the years in an attempt to control or prevent seepage losses or fluid losses to underground formations. These additives are commonly referred to as lost circulation materials. Known lost circulation materials include sawdust, gilsonite, asphalt, plastics, water soluble polymers and various thickening and gelling agents. Lost circulation materials are often ground or blended to different particle sizes based on the expected severity of lost circulation, and are intended to either plug loss zones with solids or polymers or to build up a mat to seal off the loss zone or reduce lost circulation at the loss zone.
Lost circulation materials traditionally used in attempts to control seepage losses include gilsonite, fibrous materials and calcium carbonate. However, agents such as these have not proven satisfactory.
These lost circulation materials, including known seepage loss agents, can have a variety of undesirable effects on drilling operations, including permanently damaging or plugging the oil or gas bearing formation, damaging the drilling fluid itself, and causing difficulties in maintaining the chemical or physical properties of the original drilling fluid. Additives that dissolve in the drilling fluid can alter the properties of the original fluid, such as lubricity and viscosity, which must then be corrected by additional measures. The use of additives can also cause mechanical problems in the drilling rig equipment, most notably, the fluid pumps and solids control equipment, such as shakers, screens, and centrifuges.
Some undesirable effects from solids that can affect the well bore and the drilling fluid properties, include: solids added to a hydrocarbon and water emulsion reduce the electrical stability or the emulsion stability of the drilling fluid by consuming emulsifiers and must be offset with the addition of emulsifiers to maintain the desired fluid properties; calcium carbonates with a density of 2600 kg/m3 create higher densities in the hydrocarbon drilling fluid which can increase the rate of losses; oil wetting chemicals must be added to ensure the solids are oil wet; slower rates of penetration from additional solids and higher plastic viscosities of the drilling fluid; and erosion of the deposited solids with movement of the drill string and the annular velocity of the fluid pumping action.
U.S. Pat. No. 3,455,390, assigned to Union Oil Corporation of California, discloses a method of treating a well with a water-based well treating fluid to temporarily seal off a drilled well bore without permanent loss of permeability. Finely-divided (i.e. 1 to 50 micron) oil-soluble wax particles are dispersed in the aqueous treatment fluid, which is then pumped into the drilled well to seal off the bore until the well is returned to production. The fine wax particles dissolve within 24 to 48 hours in the hydrocarbon present at oil bearing zones of the formation to allow the well to return to production. Surface active agents and emulsifiers are added to the water-based treatment fluid to aid in dispersion and control particle size. There are a number of disadvantages associated with using such a fluid.
The presence of dissolved wax, surface active agents, emulsifiers and various dissolved polymers alters the original properties of the treatment fluid and renders the fluid expensive to manufacture and maintain and thus impractical for large scale use. The fluid loss agents taught in U.S. Pat. No. 3,455,390 are not suitable for drilling with oil based drilling fluids, since they would dissolve in the hydrocarbon over a short time, contaminating the drilling fluid and potentially the oil bearing formation itself. The fine wax particles described therein would not be effective in preventing seepage losses to porous underground formations during open hole drilling but rather are more suitable for very tight formations. The use of such finely divided particles renders the fluid ineffective for controlling seepage losses. The use of fine oil-soluble particles renders the treatment fluid ineffective for preventing seepage losses, particularly when using a hydrocarbon based drilling mud.
U.S. Pat. No. 3,302,719, U.S. Pat. No. 3,593,794, U.S. Pat. No. 3,601,194, U.S. Pat. No. 3,630,280, and U.S. Pat. No. 3,684,012, also assigned to Union Oil Corporation of California, disclose similar treatment fluids comprising various combinations of fine wax, resins, polymers, copolymers, surface active agents and emulsifiers for temporarily sealing off an underground formation surrounding a drilled well bore. These fluids have the same disadvantages as set out above and are not suitable for controlling seepage losses to porous formations, particularly when using hydrocarbon based drilling muds.
Lost circulation of drilling fluid, particularly seepage loss, is one of the most serious and expensive problems facing the drilling industry today. It is, therefore, desirable to provide improved seepage loss agents, improved drilling fluids and improved methods for reducing seepage losses to underground formations during drilling operations. An improved drilling fluid and method of using same should be effective but should also be simple and economical in manufacture and use. It should also minimize damage to the formation, the drilling fluid, and the drilling equipment.